Crossed field tube having a pair of permanent magnets of different magn etomotive force



July 9, 1968 E. J. COOK 3,392,308

CROSSED FIELD TUBE HAVING A PAIR OF PERMANENT MAGNETS OF DIFFERENT MAGNETOMOTIVE FORCE Filed May 25. 1965 VARIABLE VOLTAGE IOOO I8OOV.

INJECTOR VOLTAGE '6 H62 J6 PRIOR ART 4 E 4 4 I) x (n g w .3-\' J8 5; 2 5: O U I 2- I 5 z- I {i i i .l- I .l- I

GAUSS GAUSS Fl 3 47''\ INVENTOR.

EDWARD J. COOK BY f Z ATTORNEY United States Patent CROSSED FIELD TUBE HAVING A PAIR OF PERMANENT MAGNETS OF DIFFERENT MAGNETOMOTIVE FORCE Edward J. Cook, South Hamilton, Mass., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed May 25, 1965, Ser. No. 458,617 Claims. (Cl. 315-39.63)

ABSTRACT OF THE DISCLOSURE A crossed field microwave tube is disclosed. The tube includes a cylindrical slow wave anode circuit structure concentrically surrounding a cathode electrode to define a magnetron interaction region therebetween. An electron gun is disposed at one axial end of the tube for injecting a stream of electrons axially into the interaction region for interaction with the fields of the slow wave anode circuit to produce an output microwave signal. A permanent magnet circuit produces an axial magnetic field through the magnetron interaction region and through the electron beam injection region to produce crossed electric and magnetic fields between the anode and cathode electrodes. The magnet circuit includes a pair of conically shaped permanent magnets disposed at opposite axial ends of the microwave tube for producing the axially directed magnetic field within the tube. A low reluctance magnetic shield encloses the magnets to provide a low reluctance return path for the magnetic field and also to shield the region outside of the magnet circuit from the intense magnetic field produced by the permanent magnets. The magnets produce a magnetic field profile which has a maximum intensity in the electron injection region and which progressively decreases to a uniform magnetic field intensity throughout the interaction region. In a preferred embodiment, the permanent magnets are made of platinum-cobalt material, whereby the size and the weight of the magnets are minimized. The magnetic field profile is produced by causing the permanent magnet disposed nearest to the beam injection region to have a greater magnetomotive force than the magnet on the opposite end of the tube. In a preferred embodiment, both magnets are of similar truncated conical shapes, except that the magnet producing the greater magnetomotive force has a longer axial length than the other magnet.

The optimum magnetic field required for a voltage tunable magnetron is characterized by a first region containing the electron injection gun within which the axial field intensity decreases in the axial direction taken toward the second region containing the anode to cathode interaction gap. Within the interaction gap the magnetic field should have relatively constant intensity.

Heretofore it has been proposed to form the magnetic circuit for a voltage tunable magnetron by means of employing a pair of shielded symmetrical truncated magnets for defining the useful field gap of the magnetic circuit.

Such a symmetric magnet circuit provides the desired shape inasmuch as the field intensity sags in the middle of the magnet gap to provide a first region of decreasing intensity followed by a relatively flat region of field in the center of the gap. However with the symmetric magnet circuit an unnecessary third region of field is produced wherein the field increases in the direction of the field. This undesired field region can be minimized by use of specially shaped pole pieces at opposite ends of the magnet gap. However the pole pieces use up a substantial portion of the gap length thereby requiring the magnets to be larger than necessary.

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In the present invention it has been found that the magnetic field may be shaped to conform to the optimum shape having substantially only the desired two regions, one flared and one substantially constant by making one of the magnets of substantially greater magnetomotive force than the other and properly shaping these magnets. In this manner the desired magnetic field is produced with a minimum of magnet material, whereby the size and weight of the magnetic circuit is substantially reduced.

The principal object of the present invention is the provision of an improved magnetic circuit for beam injected crossed field tubes and improved tubes using same.

One feature of the present invention is the provision of a magnetic field of the proper shape formed by a pair of magnets having substantially different magnetomotive forces, defining the magnetic gap therebetween and being surrounded by a low reluctance magnetic shield whereby size and weight of the magnetic circuit are minimized.

Another feature of the present invention is the sameas the preceding feature wherein the magnets are made of platinum cobalt whereby an intense field of high uniformity is obtained without the use of separate pole piece members.

Another feature of the present invention is the same as any one of the preceding features wherein the magnets are of truncated conical shape having substantially the same large and small diameters but being of substantially unequal length, the stronger magnet having the longer length whereby fabrication of the circuit is facilitated.

Other features and advantages of the present invention will become more apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is an enlarged longitudinal sectional view, partly schematic, of the tube and circuit employing features of the present invention,

FIG. 2 is a graph of magnetic field intensity as a function of gap length for a prior art magnetic circuit using a pair of magnets of symmetric shape and equal magnitude, and

FIG. 3 is a plot similar to that of FIG. 2 showing the field intensity for a magnetic circuit of the present invention and having a pair of magnets of unequal magnetomotive force.

Referring now to FIG. 1 there is shown the voltage tunable magnetron tube and associated magnetic circuit of the present invention. More specifically, the tube 1 comprises an evacuated stacked ceramic vacuum envelope 2 having an external microwave cavity 3 disposed within the gap 4 of a magnetic circuit formed by a pair of trunca'ed conical magnets S and 6 enclosed within a cylindrical low reluctance shield 7 as of 0.150" thick cold rolled steel. The magnetic circuit is more fully described below. Output power is extracted from the tube 1 via an ouput coaxial line 8 coupled by an RF. magnetic coupling loop 9 to the fields of the microwave cavity 3.

The tube elements within the vacuum envelope 2 will now be described in greater detail. Briefly, the vacuum envelope 2 is formed of conventional stacked ceramic configuration formed by a stack of alternating metallized ceramic ring members 11, as of alumina, and conductive electrode members, having disk like flange portions, all brazed together to form a composite envelope with the electrode structures passing through the side walls of the ceramic vacuum envelope 2. One end of the ceramic envelope 2 is closed by a ceramic plate 12 while the other end is closed by an electrode brazed into the end ring 11 of the envelope 2.

Internally of the envelope 2, the tube electrodes include a bifilar coil filamentary emitter 13 as of thoriated tungsten emitter wire disposed at one end of the envelope 2. The

emitter 13 includes a pair of leads 14 and 15 passing through and sealed into the end ceramic plate 12 in a vacuum tight manner.

A ring shaped beam injector anode electrode 16 as of copper surrounds the emitter 13 and includes a disk-like portion 17 passing out through the side wall of the envelope. A cold cathode electrode 18 as of molybdenum is coaxially disposed of the envelope 2 in axially spaced relation from the emitter 13. The cold cathode electrode 18 includes a cylindrical sole portion 19 and a flared end portion 21 which is sealed across the central opening in the end ring 11 of vacuum envelope 2.

An interdigital anode circuit 22 coaxially surrounds the sole portion 19 of the cold cathode electrode 18. The interdigital anode circuit is formed by a-pair of slotted and interdigitated flared cylinder members 23 and 24 as of copper. More specifically the cylindrical portions of each of the members 23 and 24 are slotted with an array of axial slots 25 to form an array of conductive fingers 26 in each of the cylindrical portions of the electrodes 23 and 24. The finger arrays 26 are then angularly displaced relatively to each other and then interdigitated to form the interdigital line anode circuit 22. The circuit 22 is supported by means of the disk-like flange portions 27 and 28 of the cylindrical members which flange portions extend outwardly of and through the envelope 2 to the external microwave circuit 3. The external circuit is formed by an annular conductive channel 29 as of copper which is of generally toroidal shape. The channel 29 is aflixed to the disk like flange portions as by silver soldering at the mating edges of the disk flange member portions 27 and 28 to form a rigid tube structure. The channel 29 is apertured at 31 to receive the coaxial output line 8 and loop 9.

Operating potentials are applied to the tube 1 from suitable power supplies. More particularly, a small cathode heater voltage is derived from battery 33 and fed across the directly heated cathode emiiter filament 13 via leads 34 and 35, respectively, which include separate electrode portions 34' and 35' brazed across the ceramic end plate 12 of the envelope 2 which in turn connect to filament leads 14 and 15, respectively. The negative potential side of the cathode filament 13 is also connected via insulated lead 36 to the cold cathode electrode 18. The lead 36 passes between the outside wall of the toroidal microwave circuit 3 and the inside wall of the magnetic shield 7. The toroidal microwave circuit 3 is made to have a slightly smaller outside diameter than the inside diameter of the shield to allow suflicient clearance for the lead 36.

An injector anode voltage supply 37 as of 525 volts positive with respect to the cathode emitter 13 and electrode 18 is connected to the injector electrode 17 via insulated lead 38. A variable positive anode voltage supply 39, as of 1000 to 1800 volts relative to the injector voltage, is applied to the anode circuit 22 and interconnected mag netic circuit via grounded lead 41. A pair of insulator sheet members 42 as of 0.015" thick Teflon are disposed between the magnets and 6 and the end electrode portions of the tube envelope 2 for holding off the applied anode to cathode voltage, as of 1500 to 2300 volts. The tube envelope 2 with fixedly attached external microwave circuit 3 is fixedly held in position within the surrounding magnetic circuit by means of a solid epoxy potting material such as Stycast No. 1090 made by Emmerson Cummings and introduced, in fluid form, into the voids 44 between the tube envelope 2 and the enclosing magnetic circuit members 5, 6 and 7 via suitable holes, not shown, in the cylindrical shield 7. The potting material is then allowed to set up to form a solid rigid tube structure.

In operation, electrons emitted from emitter 13 are formed into a tubular stream and injected axially into the annular electronic interaction region 45 via the combined action of the radial electric fields between the electrodes 13, 17, 18 and 22 and the axial magnetic field produced by the magnetic circuit. The electrons circulate around the sole 19 of the cold, cathode electrode 18 cumulatively interacting with the RF. fields of the interdigital line to produce backward wave oscillation on the anode circuit 22. Power is extracted from the circuit 22 and fed to a load via output loop 9 and terminal 8. The output frequency depends upon the anode to cathode voltage and thus the tube 1 is electronically tuned by varying the voltage of voltage supply 39.

The optimum magnetic field in gap 4, as previously pointed out, is characterized by having a constant intensity in the electronic interaction gap 45 while flaring to increased intensity in the injector region 46.,This shape of field properly introduces the stream of electrons into the interaction region and yields optimum efliciency.

The prior method for obtaining the desired field shape in the injector and interaction regions is illustrated in FIG. 2 and comprised use of two magnets 5 and -6 polarized in the same direction and of equal magnetomotive force. Such a magnetic circuit produced a satisfactory flared intensity in the injection region 46 but produce a variance of about gauss, at about 2000 gauss, over the length of the interaction region 45. This is a substantial variance from the desired constant intensity of fieldover the interaction region 45. In addition, this prior magnetic circuit includes the second flared intensity region 47 which is as large as the injector flared region and which third region 47 is not serving any useful purpose. Thus the second flared region represents wasted magnetomotive force and thus requires more magnet weight and size than necessary.

Referring now to FIG. 3 there is shown the magnetic field intensity plot for the magnetic circuit of FIG. 1 wherein the magnetomotive force of the injector magnet 5 is substantially greater than the magnetomotive force of the cold cathode magnet 6. In this case the desired shape of field is produced having the flared intensity injection region 46 and uniform intensity interaction region 45 without the necessity of producing the third flared intensity region as encountered in the prior art. Accordingly, in the improved field shaping circuit of FIGS. 1 and 3, the magnet gap length can be reduced to a minimum thereby conserving magnetomotive force and thereby reducing the size and weight of the magnetic circuit.

In a typical example of a magnetic circuit of the present invention as used for a voltage tunable magnetron tube 1 operable over the range from 2.6 to 3.7 gc. with power output of 12 watts the gap length is about 0.700", the magnets are of equal diameter for ease of construction having maximum diameters of 1.300" and minimum diameters of 0.875" with the larger magnet 5 having an axial length of 0.500" and the smaller magnet having a length of 0.310". The magnets 5 and 6 were made of platinum-cobalt and provided a uniform magnetic intensity of 1950 gauss over the interaction region 45 which was 0.200 long with only a 40 gauss variance between maximum and minimum field intensities within the interaction region 45. The tube 1 including its magnetic circuit and integral heat sink, not shown, was only 1.8" in diameter, 1.7" long and weighed only 19 oz.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accom-- microwave circuit means for electromagnetic interaction between RF. energy on said circuit means and electrons of the stream; means for extracting output microwave energy from said circuit means; means forming a magnetic circuit for producing a DC. magnetic field threading through both said injection and electronic interaction regions of space; said magnetic field having an intensity in one of said regions substantially greater than in said other region; said magnetic circuit means including, a pair of permanent magnets spaced apart to define a magnetic gap therebetween which contains the DO magnetic field threading through both said regions of space, and said permanent magnet of said pair of permanent magnets which is closest to said region of most intense field having a substantially greater magnetomotive force than the other permanent magnet of said pair of magnets, whereby the size and weight of said magnetic circuit is minimized.

2. The apparatus according to claim 1 wherein said region of space which contains the more intense D.C. magnetic field threading therethrough is said injector region of space, and said permanent magnet which is closest to said injector region generating the greatest magnetomotive force.

3. The apparatus according to claim 2 wherein said magnets are made of platinum-cobalt.

4. The apparatus according to claim 2 wherein said pair of magnets are of truncated conical shape differing substantially only in axial length and with said permanent magnet of greater magnetomotive force having a longer axial length than said other permanent magnet of lesser magnetomotive force.

5. The apparatus according to claim 2 including means forming a magnetic shield of relatively low reluctance material surrounding said pair of magnets and said magnet gap therebetween and providing a relatively low reluctance return path for magnetic field lines between said magnets but oppositely directly to the field lines in the magnet gap.

6. A microwave tube apparatus including means forming a hollow cylindrical dielectric evacuated envelope, means forming a thermionic cathode emitter at one end of said envelope, means forming a cold cathode sole electrode projecting axially of and into said envelope from the other end thereof, means forming an anode circuit surrounding said sole electrode and passing through the cylindrical side portions of said envelope to define an internal anode circuit portion and an external anode circuit portion external of said vacuum envelope, means forming an injector anode electrode surrounding said cathode emitter, means forming a pair of spaced permanent magnets disposed at opposite ends of said cylindrical envelope and defining a magnetic gap therebetween with the magnetic field lines of said magnet gap being axially directed of and threading through said cylindrical envelope, said pair of permanent magnets being energized and shaped such that one permanent magnet produces substantially greater magnetomotive force than the other permanent magnet of said pair, and the magnet having the greater magnetomotive force being disposed at the emitter end of said vacuum envelope to produce a magnetic field shape having greater intensity in the cathode emitter region than in the region between said cathode sole and said anode circuit, and means forming a magnetic shield surrounding said magnetic gap and said pair of magnets.

7. The apparatus according to claim 6 including a dielectric potting material filling the spaces between said vacuum envelope and said surrounding shield means for rigidly holding said vacuum envelope in place.

8. The apparatus according to claim 6 wherein said magnets are magnetized in an aiding direction and have truncated conical shapes.

9. The apparatus according to claim 8 wherein said magnet having the greater magnetomotive force is substantially longer in axial extent than the other magnet of said pair.

10. The apparatus of claim 9 wherein said magnets are made of platinum cobalt.

References Cited UNITED STATES PATENTS 2,473,547 6/1949 Schmidt BIS-39.71 2,939,046 5/1960 Wilbur 3l539.71 3,229,152 1/1966 Hodges 3l539.71 3,302,060 1/1967 Blok et al 31539.71 X

HERMAN KARL SAALBACH, Primary Examiner.

P. L. GENSLER, Assistant Examiner. 

